RFR: 8311867: StructuredTaskScope.shutdown does not interrupt newly started threads

[Jaikiran Pai]

On Wed, 12 Jul 2023 11:56:18 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> test/jdk/java/util/concurrent/StructuredTaskScope/StressShutdown.java line 83:
>> 
>>> 81:             // fork subtask to shutdown
>>> 82:             scope.fork(() -> {
>>> 83:                 scope.shutdown();
>> 
>> Hello Alan, the proposed source change in `ThreadFlock` and the reason for that change appear good to me.
>> However, as far as I understand, it appears to me that this test may not be reproducing the issue where shutdown gets called when there are threads that are about to start. From what I see in the API and implementation of `scope.fork()`, when the `fork()` returns, it's guaranteed that a `Thread` for the subtask has been started (keeping aside the failed to start cases). So in this test here, when we reach this point where we attempt a `shutdown()` all 15 "beforeShutdown" subtasks would already have threads that are started and alive. i.e. there won't be any "about to be started thread". As for the 15 "afterShutdown" forks() that follow, they would already notice that the scope is shutdown and won't start the new threads.
>> Did I misunderstand this test?
>
>> As for the 15 "afterShutdown" forks() that follow, they would already notice that the scope is shutdown and won't start the new threads.
>> Did I misunderstand this test?
> 
> The test arranges for a subtask to do shutdown, it's not done by the main task. That setups up the conditions for the shutdown to race with the subsequent forking done by the main task.

Ah! Indeed, I missed that crucial detail. So the shutdown() call itself can happen when one of the "afterShutdown" subtask is being forked and a new thread being created for it. Makes sense. Thank you for that explanation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14833#discussion_r1261073745